I was working for Amtrak during that era and the MBTA had the final say on what operated where. Some of the high maintenance equipment was moved to the North Station routes as there was an enclosed site where they could be maintained. Also, the passenger volumes out of South Station were astronomically higher than out of North Station, so longer trains, or higher capacity - bilevel equipment- were necessary. Some North Station lines could not use bilevels until some clearances had been improved.

Note that during the summer of 1977, just after the MBTA bought all the B&M's commuter routes and equipment, they removed all the underbody housings protecting the diesel prime movers. This is why the winter of 1978 reduced almost all of them to a single functioning prime mover which idled to produce electricity and heat for the car.

The MBTA acquired several groups of serviceable RDCs after the B&M puchase - exx-NH and exx-NYC units from Conrail in 1978, several ex-CPR between 1976 (per Duke & Keilty) and 1985, several exx-RDG, ex-SEPTA in 1985 and one from B&O in 1984. They also leased 9 from NJT (mostly exx-P-RSL) in 1988.

Doug Kydd discusses late uses of RDC in his autobiography, but I'll have to figure out where I filed it to tell you the title/publisher.

So this is way OT but on April 6, 1993, a CP Rail RDC2 #91 (CP's Railway Technology Exhibit Car) showed up at Ayer and Charlestown's Moran Intermodal terminal as part of a special Guilford business train to inaugurate joint CP-GRS container service to Moran terminal. Here's a 19+ minute video posted on Youtube by "nhrrman" that documented the event, including the scene at Ayer when GRS and CP/D&H separated their equipment to go home their separate ways. At the very end of the video, we see a freshly painted CP dual flags SD40-2 and D&H GP38-2 pulling a train of empty open top auto racks west from Ayer with CP RDC2 #91 acting as the caboose. Not sure if 91 retained its propulsion system when it was converted from revenue passenger service to Exhibit car in 1978